Electric wave filter



Oct. 17, 1939.

E. c. CORK ET AL ,211

' ELECTRIC WAVE FILTER Filed Nov. 9, 1957 2 Sheets-Sheet 1 M/VE/V T025FDWARD C. (021? JOSEPH L. PAWSE) A TTORNEY t-17,1939. E. Gm HAL 2116,211

ELECTRIC WAVE FILTER Filed Nov. 9, 1937 2 Sheets-Sheet 2 INVENTORSEDWARD C- CORK JOSEPH L- PAWSE' Y A TTORNEY Patented Oct. 17, 1939UNITED STATES PATENT OFFICE ELECTRIC WAVE FILTER Middlesex, England,

Britain a company of Great Application November 9, 1937, Serial No.173,552 In Great Britain November 18, 1936 7 Claims.

This invention relates to electric Wave filters.

It is the chief object of the present invention to provide improvedfilters which are specially suitable for reducing interference caused byvery high frequencies (such as television frequencies) in lowerfrequency circuits.

For example, it is desirable in direct current leads or in low frequencyalternating current leads, to prevent such leads from transmittinginterfering signals of the very high frequencies mentioned above. It isalso desirable to provide suitably designed filters in interstagecouplings, for example, in the grid lead of a thermionic valve, used inamplifiers or modulators, in order to prevent interference by highfrequency signals. In addition, in cables employed for connecting atelevision camera to a transmitter, it is necessary to prevent signals,for example, of the carrier frequency which is usually about 44megacycles, from interfering with the signals transmitting along thecable.

The present invention provides filters suitable for these and otherpurposes.

According to one feature of the invention, the filtercomprises a seriesreactance of high impedance to a short wave interfering signal and ashunt reactance of low impedance to said signal, at least one of saidreactances comprising a shielded inductance self-tuned to the unwantedfrequency mainly by virtue of the length of wire employed.

According to another feature of the invention, an electric filter isprovided, including a series and a shunt reactance, said series andshunt reactances being mounted within shields connected together in sucha manner that the said components and the shields constitute a unitarystructure whereby the filter can be readily applied to a lead or cable.

In order that the said invention may be clearly understood and readilycarried into effect the same will now be more fully described withreference to the accompanying drawings in which:

Figure 1 illustrates a filter suitable for use in direct current oralternating current supply leads,

Figures 2 and 3 illustrate filters of low capacity and suitable for usein interstage couplings,

Figure 4 illustrates a filter mainly for use in high frequency cables,and

Figure 5 illustrates more in detail the type of filter shown in Fig. 2.

As shown in Figure 1, the filter comprises an inductance 5 and two shuntcapacities 6 and l, the inductance 5 being mounted within a shield 8which is in the form of a tube the ends of which are closed byinsulating members and through which project the ends of the conductorof the inductance 5. In this example and in the following examples theinductances may be wound on insulating bobbins the ends of which fit theinternal diameter of the shields. This type of filter is designed foruse in the direct current supply leads or alternating current leads oflow frequency, such as cycles, for the purpose of preventinginterference at high frequencies. The inductance 5 is tuned to theunwanted frequency by virtue of the length of wire employed which isapproximately a quarter of the wavelength of the unwanted frequency andthus provides a high series impedance to the unwanted frequency, whilstthe condensers 6 and 8 are shunts of low impedance to the interferingsignal. In one form of filter designed to prevent interference at 44megacycles, the inductance 5 may be composed of a length of enamelledcopper wire (22 S. W. G.) about 5 feet 10 inches long which is thusapproximately of a length equal to a quarter of the wave length of theinterfering signal and is wound in 51 turns on a former of of an inchdiameter, giving an overall length of about 1%; inches; the inductanceof the coil so formed is about 4.2 microhenries. The condensers 6 and 8may each have a capacity of .002 microfarads.

Figure 2 of the drawings illustrates a filter suitable for use as a lowcapacity filter for an interstage coupling. This filter is a T networkand comprises two series inductances 9 and I0 and a series tuned shuntreactance composed of an inductance ll and a condenser 12. Thecomponents shown in Figure 2 are shielded by the provision of shieldsl3, l4 and 15, these shields being of tubular form and being connectedat their inner ends to a hollow junction box [5a. The ends of thetubular shields are connected in any suitable way to the junction box150, as by screw threading so that the complete assemblage forms aunitary structure. Such a construction is particularly suitable wherethe filter is designed for use in conjunction with concentric cablescomposed of an inner conductor airspaced from a sheath constituting areturn conductor. In this case, the shields may have an externaldiameter corresponding to the internal diameter of the sheath so thatthe filters can be readily connected to the cable in a telescopicmanner. The shield 15 will, of course, in most cases be connected toearth. The inductances 9 and H], in one example, are tuned by virtue ofthe length of wire employed and the selfcapacities and that due to thescreens to resonate at 44 megacycles, the series tuned shunt circuitbeing arranged to provide a by-pass to earth at the above mentionedfrequency. The condenser [2 may have a capacity of 6 micromicro-farads,whilst the inductance H and 44 megacycles is arranged to have aninductance of 2 microhenries. The filter shown in Figure 2 is especiallysuitable for use in conjunction with a high termination impedance, suchas the input impedance of the grid of a valve.

Several filters similar to that shown in Fig. 2 and in the subsequentfigures can be connected in cascade.

The filter shown in Figure 3 is similar to the filter shown in Figure 2,with the exception that it is arranged to have less series inductance attelevision picture frequencies. In this case, two smaller seriesinductances l6 and IT are employed associated with a series tuned shuntpath similar to that shown in Fig. 2, but associated with eachinductance it and H is a further inductance i3 and it each of which maybe connected at one of its ends to one of the ends of the inductances i6and H as shown. The inductances l8 and I9 are tuned by virtue of thelength of wire employed to resonate at the undesired frequency, which,in one example, may be 44 megacycles. The coupling between the pairs ofinductances I6 and I8 and I! and I5 are such that at televisionmodulation frequencies the series inductance is less than in the case ofFigure 2, whereas at the interfering frequency the required highimpedance to prevent transmission of such frequencies is present.

Figure 4 shows a somewhat similar type of filter of the unitaryconstruction shown in Figures 2 and 3, but in this case, the filter isdesigned particularly for use in conjunction with a cable connecting atelevision camera to a transmitter. In this case the two seriesinductances 2t and 2| are associated with a shunt condenser 22. Thethree components are shielded, as previously described and theinductances 20 and 2! are tuned to the unwanted frequency by theirself-capacities and the shunt capacities of the shields and virtue ofthe length of Wire employed, which will be approximately of a lengthequal to a quarter of the wavelength of the unwanted frequency which, ina particular instance, is 44 megacycles. The Value of the condenser 22may be .0002 microfarad and with such an arrangement the filter willfunction as a low-pass filter having very high attenuation at 44megacycles, the theoretical cut-off frequency being of the order of 10megacycles. The filter shown in Figure 4 may be employed for terminatingthe television camera cable and will be used in series with the line. Atmodulation frequencies the filter will act as a slight extension of alow attenuation line of the same characteristic impedance, whilst at 44megacycles which is the frequency of interfering signals, the filterwill produce heavy attenuation.

It will be observed from Figs. 1-4 that the shields surround theirassociated components very closely and thus serve substantially toeliminate direct capacity between the individual reactances. It is foundthat the use of shields closely surrounding the components is far moreeffective in reducing direct capacity between the components than theuse of a screening box of the usual rectangular form which houses allthe compo nents.

Figure 5 illustrates more in detail the mechanical construction of afilter similar to that shown in Figure 2 and also shows the filterconnected to a cable and constructed for connection to the main screenof an amplifier or similar device. The parts in Figure 5 correspondingto the equivalent parts in Figure 2 bear the same reference letters and,as shown, the series inductances 9 and Ill and the shunt inductance Hare wound on bobbins 23, 24 and 25 which fit the internal diameter ofthe shields l3, l4, and l5,these shields being screw-threaded into thecommon junction box l5a which is of cubic form. The leads from theadjacent ends of inductances 9, l and II are connected together in theinteror of the box 15a and in order to afford the required connectionboth in the interior and the box 15a and at the opposite ends of theinductances 9 and Ill the bobbins 25 are provided with rigid conductingrods projecting therefrom the. wire of the inductances being passedthrough holes in the bobbins and wound around the rigid conductors andsoldered thereto. The rigid rods which project in the interior of thebox I after the bobbins are assembled as shown substantially abutagainst one another and are then soldered together. To enable thissoldering to be effected. the top of the box 55a is provided with aremovable cap- 26 permitting access to the interior of the box We. Thecondenser I2 is conveniently constituted by providing on the lower endof the bobbin 25 as shown a conducting disc 21 to which the lower end ofthe inductance H is connected the other plate of the condenser beingconstituted by a disc 28 having a depression 29 therein, the discs 2'!and 28 being separated by a disc of mica 3E3 constituing the dielectricof the condenser. The provision of the depression 29 permits ofadjustment of the capacity of the condenser and for adjusting purposesthe two plates of the condenser are urged together by a screw 3ithreaded into the interior of the shield l5, the lower end of the shieldbeing closed by a further screw or cap 32.

In the embodiment shown in Figure 5, in order to connect the filter to acable and to the main screen of an amplifier or similar device, there isprovided at the end of the screen l3 remote from the end secured to thebox 55a a flange 33 between which and a further flange 3 3 is clamped aplate 35 which is adapted to be secured to the main screen 36 of theamplifier or similar device. Associated with the flange 34 is a cap 31,the cap 31 and the flange 34 having formed therein a channel 38 in whichthe sheath 39 of a cable can be clamped, the centre conductor 40 of thecable being connected to the end of the inductance 9. When securing thecable to the filter the cap 31 is removed so that the centre conductorcan be soldered or otherwise connected to the rigid conductor projectingfrom the bobbin 23 and thereafter the cap 3'! is secured in position byscrews (not shown) and thereby clamping the sheath of the cable betweenthe cap 31 and the flange 34. A similar arrangement may be employedwhere the filter is used as an interstage coupling in an amplifier.

Where the filter is employed to connect a television cable to anamplifier the shield l3, for example, is arranged to fit into theinterior of the lead sheath of the cable the inductance 9 beingconnected to the centre conductor of the cable. The shield M may, inthis case be arranged to be received in a suitable socket in electricalcontact with the main screening casing of for example an amplifier theinput and output leads of the filter being thus disposed on oppositesides of the main screening casing of the amplifier.

We claim:

1. An electric wave filter comprising a series reactance of highimpedance to a short wave interfering signal and a shunt reactance oflow impedance to said signal, shields for both of said reactances, oneof said reactances being an inductance self-tuned to the unwantedfrequency mainly by virtue of the length of wire employed.

2. An electric wave filter according to claim 1, wherein the filtercomprises a T-network the series and shunt reactances being mountedwithin shields connected together by a. common junction box.

3. An electric wave filter comprising at least a series and a shuntreactance, and individual shields for both of said reactances, saidseries reactance comprising a first inductance and a second inductancecoupled thereto to reduce the series inductance at a predeterminedfrequency.

4. An electric wave filter according to claim 1, wherein the shields areof circular cross-section.

5. An electric wave filter according to claim 1,

wherein the series reactances comprise inductances self-tuned to theunwanted frequency mainly by virtue of the length of wire employed.

6. An electric wave filter including an inductance and capacity, abobbin on which said inductance is wound, a conducting element providedat one end of said bobbin and in electrical contact with saidinductance, a further conducting element separated from said firstelement by a member of dielectric material, a depression in one of saidconducting elements, a shield surrounding said bobbin and means screwthreaded in said shield for adjusting the capacity formed between saidconducting elements.

'7. An electrical wave filter according to claim 1, wherein one of saidshields is provided with a flange at one end associated with a junctionbox so constructed as to enable a cable to be connected to the filterthe said flange and junction box enabling the filter to be connected toa conducting screen such as a screening division or box of an amplifier.

EDWARD CECIL CORK. JOSEPH LADE PAWSEY.

